This invention relates to motor control systems and, more particularly, to a top speed limiting arrangement for incorporation in a DC motor control system.
Motor control systems are well known in the prior art for operating a DC motor from an AC supply at preselected speeds. Typically, such systems utilize phase control circuits connected between the DC motor and the cyclically varying AC power supply. A firing circuit is provided which responds to speed selection, typically via a potentiometer, for firing the phase control circuit at the appropriate phase angle of the power supply so as to control the motor speed. In particular, the phase control circuit typically includes a silicon controlled rectifier (SCR) serially connected betweeen the power supply and the motor and the firing circuit includes a charging capacitor connected to the gate electrode of the SCR so as to trigger the SCR upon reaching a predetermined voltage. Speed control is then attained by controlling the charging rate of the capacitor. Such systems typically further include some feedback from the motor for purposes of regulating the speed so that the selected speed is maintained irrespective of variations in line voltage and/or motor load.
Such prior art motor control systems commonly experienced difficulties in holding the maximum opeerating speed because of the many variables involved, i.e., variations in initial machine torques, changes in machine torque with time, variations in line voltages, etc. In certain applications, it is critical to keep the maximum motor speed below some upper limit. For example, in a sewing machine having electronic pattern control, the response time of the servo systems for the bight and feed actuators sets the upper limit on the speed of the sewing machine main drive motor.
It is therefore an object of this invention to provide a motor speed control system with an arrangement for limiting the top speed of the motor.
It is the further object of this invention to provide such an arrangement wherein desired speed settings below this upper limit are not affected.